fix: migrate to stackvox 0.4.x unified CLI

[StuBehan]

Summary

stackvox 0.4.x consolidated its CLI — there is no separate `stackvox-say` console_script anymore; speech goes through `stackvox say ` as a subcommand. `notify.sh` and `Speaker.swift` both still pointed at the old binary, so on fresh installs (where only `stackvox` exists in the venv) the existence guard bailed silently and voice never fired.

Closes #14, reported by @OMauriStkOne.

Failure mode this fixes

• Install looks healthy, daemon running, models downloaded
• `stackvox say "..."` works manually
• `STACKNUDGE_VOICE=true` set
• Hook fires → banner + sound play → voice never speaks, no error surfaced

Changes

`notify.sh`

• Drop `STACKVOX_SAY` variable
• `speak_notification` invokes `"$STACKVOX" say ...`
• `voice_phrase_for` gates on `STACKVOX` existence
• New `nudge_debug()` helper writes to stderr when `STACKNUDGE_DEBUG=true`. Logs the missing-binary and missing-daemon-socket cases that previously failed silently — so the next time something like this happens, users can diagnose with one env var instead of bisecting the script.

`panel/Speaker.swift`

• Same migration on the Swift side (used by the menu-bar voice toggle preview and the voice-cycle preview in Settings)

Test plan

• `stackvox say --voice af_aoede ...` runs cleanly against an installed venv
• `notify.sh claude-code stop` with `STACKNUDGE_VOICE=true` completes the voice path without error
• `STACKNUDGE_DEBUG=true` surfaces the diagnostic when stackvox is absent
• `shellcheck -S warning` and `bash -n` clean
• `make build` (swiftc path) and `swift build` (Package.swift) both succeed

Out of scope (deliberate)

• The two related issues from the same install session (install.sh fails silently when system Python is < 3.10 (error swallowed by Swift build warnings) #11 install.sh silent failure on Python < 3.10, install.sh leaves stale hook entries when upgrading from a previous install path (e.g. tinynudge → stack-nudge) #12 stale hook entries on upgrade) are independent of this — covered by separate PRs.

🤖 Generated with Claude Code

[StuBehan]

Confirmed against stackvox 0.4.0 on PyPI — stackvox say --voice ... --speed ... --lang ... is unchanged from 0.3.x (0.4.0 only adds a per-user TOML config layer for defaults). Force-pushed to bump install.sh's pin from stackvox>=0.3.0 to stackvox>=0.4.0 so we're explicit about the version we've validated against.